(1) Field of the Invention
The present invention relates to a direct-heated gas-flow measuring apparatus having film resistors which serve as temperature detecting means as well as electric heaters. Such a direct-heated gas-flow measuring apparatus can be used, for example, for measuring the flow rate of engine intake air.
(2) Description of the Related Art
Generally, in an internal combustion engine, the amount of intake air is one of the most important parameters for controlling the fuel injection amount, ignition timing, and the like. A gas-flow measuring apparatus, i.e., an airflow meter is provided for measuring the same. One of the more common prior art airflow meters is the vane-type, which is, however, disadvantageous in scale, response speed characteristics and the like. Recently, airflow meters having temperature-dependent resistors have been developed, which are advantageous in scale, response speed characteristics and the like.
There are two types of airflow meters having temperature dependent resistors, i.e., the heater-type and direct-heated type. The heater-type airflow meter may consist of an electric heater resistor provided in an intake-air passage of an engine and two temperature-dependent resistors arranged on the upstream and downstream sides of the electric heater resistor. In this case, the temperature-dependent resistor on the downstream side is used for detecting the temperature of air heated by the heater resistor, while the temperature-dependent resistor on the upstream side is used for detecting the temperature of non-heated air. The current flowing through the heater resistor is controlled for constant difference in temperature between the two temperature-dependent resistors, thereby detecting the voltage applied to the heater resistor as the mass flow rate of air.
In this heater-type airflow meter, if no upstream temperature-dependent resistor is provided and the current of the heater resistor is controlled for a constant temperature of the downstream temperature-dependent resistor, the voltage applied to the heater resistor is detected as the volume flow rate of air.
On the other hand, the direct-heated type airflow meter may consist of a film resistor which serves not only as an electric heater, but also as a temperature-detecting means for detecting the temperature of the heated air. Also, the direct-heated type airflow meter may consist of a temperature-dependent resistor for detecting the temperature of non-heated air. Thus, the current flowing through the film resistor and the temperature dependent resistor, thereby detecting the voltage applied to the film resistor as the mass flow rate of air. In this direct-heated type airflow meter too, if no temperature-dependent resistor is provided and the current of the heater resistor is controlled for a constant temperature of the film resistor, the voltage applied to the film resistor is detected as the volume flow rate of air.
A conventional direct-heated airflow meter includes a small measuring tube (duct) in an intake air passage with a single film resistor at the center of the duct. However, since the air flow rate distribution is subject to the air flow rate, the configuration of the intake air passage, the position of the duct, etc., the airflow rate distribution is not always uniform. In other words, the air flow rate within the duct is not always representative of the airflow rate within the intake air passage.
For obtaining the optimum airflow rate for the intake air passage, it has been considered to extend hot wires polygonally over the cross-section of the intake air passage, thus obtaining a mean airflow rate within the intake air passage. In this case, however, the heat mass of the sensing portion, i.e., the hot wires, is so large as to remarkably reduce the response characteristics.